1. Field of the Invention
The present invention relates to an output buffer circuit for transmitting a logical signal to a transmission line and, in particular, to an output buffer circuit, a differential output buffer circuit, an output buffer circuit having a regulation circuit and a regulation function and a transmission method, capable of improving resolution of a pre-emphasis amount of, particularly a CMOS output circuit, by applying a pre-emphasis to a transmission output waveform for the purpose of compensating for signal attenuation of the transmission line.
2. Description of the Related Art
An electronic circuit apparatus, requiring an operating speed which has become higher year after year, has been actively researched and developed to implement the higher operating speed. The high-speed electronic circuit apparatus provides advantages such as short-time implementation of even the processing for which conventionally it had taken much time or the processing which had been thought to be impossible. In addition, the high-speed electronic circuit apparatus can reduce processing cost and contribute to improvement of services in the world. Further, to manufacture such an excellent apparatus, the industrial world becomes more activated.
For implementation of such a high-speed electronic circuit apparatus, there have increased demands of high speed transmission of data transmission signals between LSI internal circuits which are components of an electronic circuit apparatus, between LSIs, between printed circuit boards, between apparatuses and between housings.
As one of high-speed techniques of data transmission, for example, there has been known a technique of preventing generation of garbled data due to reflected waves by disposing a termination resistance matched with a characteristic impedance of a transmission line at a signal receiving unit to prevent reflection of a received signal, as disclosed in Japanese Patent Laid-Open Publication No. 2003-309461.
Further, to compensate for signal attenuation in LSI internal wiring, LSI package wiring, printed circuit board wiring and a transmission line represented by a cable or a connector, there has been practically used an output buffer circuit with a pre-emphasis function which increases a signal amplitude of a high-frequency component of a signal or reduces a signal amplitude of a low-frequency component.
In addition, a CMOS-type output circuit has been used to increase a processing volume of an electronic circuit apparatus by increasing the number of circuits for connecting between LSI internal circuits, between LSIs, between printed circuit boards, between apparatuses or between housings without increasing power consumption.
Referring to FIG. 8, a pre-emphasis will be described below. The pre-emphasis highlights an edge of a waveform to compensate for damage to a printed circuit board wiring. It is shown that presence or absence of a pre-emphasis highlights an edge of a driver waveform or a receiver input waveform.
The resolution of the pre-emphasis requires approximately 1/48 in view of automatic adjustment of an emphasis amount and a CMOS-type output circuit has the following drawback: a circuit scale increases with resolution. FIGS. 9A and 9B illustrate examples of resolutions of 1/24 and 1/48.
FIGS. 10A and 10B illustrate circuits in emphasis resolutions of ⅕ and 1/10. It is understood that an emphasis resolution of 1/10 enlarges a circuit scale. FIG. 10C is an example of the present invention and detailed description will be made later, which illustrates that even an emphasis resolution of 1/10 does not cause so large circuit scale. An output impedance deviates from a characteristic impedance of a transmission line, but a circuit is divided into five portions, which can control a difference to several Ω or less.
Japanese Patent Laid-Open Publication No. 2007-251469 proposes an output buffer circuit or the like, which is formed with a selector for switching to prevent an output impedance from being changed by an emphasis amount.
In a technique disclosed in Japanese Patent Laid-Open Publication No. 2003-309461, an output impedance may be largely different from a characteristic impedance of a transmission line, depending upon an emphasis amount. In that case, waveform quality is deteriorated by a reflected wave.
A technique disclosed in Japanese Patent Laid-Open Publication No. 2007-251469 uses a selector to prevent an output impedance from changing due to an emphasis amount by switching and makes variable a resistor for setting the output impedance to the same as a characteristic impedance of a transmission line and therefore five resistors generally have required to obtain the same value or to change, keeping the same ratio.